1. Field of the Invention
This invention relates to a low pressure, low temperature process for production of high purity methanol from near stoichiometric ratios of hydrogen and carbon monoxide and an improved double tube reactor therefor.
2. Description of the Prior Art
Until a high pressure/high temperature methanol synthesis technology was developed by BASF in Germany in 1923, distillation of wood was the only commercially significant method to produce methanol. The high pressure/high temperature technology employed a synthetic route using pressurized gas mixtures of H.sub.2, CO, CO.sub.2 and CH.sub.4 in the presence of Zn-Cr based catalysts. The pressurized gas mixtures for methanol synthesis were derived from mixing steam with gaseous, liquid or solid hydrocarbon feedstocks, and preheating to 425.degree. to 550.degree. C. before feeding to a reformer. Very high pressures, typically 300 to 350 atmospheres, were applied in order to obtain a reasonable conversion at the high operating temperatures of the Zn-Cr based catalysts (320.degree. to 380.degree. C.) where the methanol synthesis equilibrium constraints are poor.
In the 1960's, highly active and durable copper-zinc oxide based catalysts were developed for methanol synthesis. These catalysts were so active that the methanol synthesis process could operate at much lower temperatures, 200.degree. to 300.degree. C., than the prior processes and permitted the use of lower operating pressures, 50 to 150 atmospheres. By the late 1970's, most of the methanol synthesis plants in the United States used low pressure technology due to the advantages of lower compression costs, reduced byproduct formation, longer catalyst life, and lower capital costs. The major differences among these low pressure methanol synthesis processes were in the methanol reactor designs used to remove the heat generated by the highly exothermic methanol synthesis reaction and the reformer configurations: one-stage or two-stage reforming. All of the modern low pressure technologies have required a large compressor to bring the process gas to the methanol reactor operating pressure and a step to remove the compressor oil before the methanol synthesis loop. Also, the raw methanol produced from these processes contained approximately 25 mole % water and impurities such as dimethyl ether and higher alcohols. Therefore, a methanol purification step of stripping columns, distillation columns, and the like was needed in order to achieve the required methanol purity.
Several United States patents teach the reaction of H.sub.2 plus CO to form methanol. U.S. Pat. No. 4,122,110 teaches the reaction of H.sub.2 and CO in the presence of a catalyst having at least four metallic components to form linear saturated primary alcohols, the selectivity of C.sub.2 or more often being higher than 70% by weight. Several patents teach removal of CO.sub.2 from process gas obtained from a hydrocarbon/steam reforming process prior to reaction of H.sub.2 and CO in a methanol forming reactor: British patent 1,159,035 teaching CO.sub.2 maybe removed completely from the synthesis gas, but part of the removed CO.sub.2 is added to the feed to the methanol reactor using a catalyst containing CuO and ZnO and at least one other difficultly reducible Group II to IV metal oxide; U.S. Pat. No. 4,348,487 teaching production of methanol by catalytic coal gasification wherein CO.sub.2 is removed from the process gas and then reintroduced back into the methanol synthesis zone feed in order to activate the methanol synthesis catalyst; U.S. Pat. No. 3,962,300 teaching a process for producing methanol using a partial oxidation treatment followed by methanol formation by contacting with a copper-containing catalyst which is indirectly cooled with water boiling under superatmospheric pressure resulting in the production of high pressure steam which is expanded by generating power to produce compression energy for the gases to be compressed in the process, thereby recognizing the problem of compression energy in the methanol production process; and U.S. Pat. No. 4,013,454 teaching partial removal of CO.sub.2 in a simultaneous production of methanol or ammonia and again recognizing the problem of CO.sub.2 and compression energy in the methanol synthesis process.
A number of patents relating to methanol synthesis recognize that CO.sub.2 and H.sub.2 O are in the product methanol and must be removed by downstream processes to obtain high purity methanol: U.S. Pat. Nos. 3,501,516; 3,993,457; 4,048,250; and United Kingdom patent application 2142331A.
The use of Cu-Cr.sub.2 O.sub.3 as a selective catalyst for methanol production without the requirement of CO.sub.2 for catalyst activity promotion has been recognized in Monnier, J. R., Apai, G., and Hanrahan, M. J., "Effect of CO.sub.2 on the Conversion of H.sub.2 /CO to Methanol over Copper-Chromia Catalysts", Journal of Catalysis, 88, pg. 523-525 (1984). The characterization and catalytic activity for methanol formation using promoted Cu-Cr oxide catalysts is taught by J. Monnier and G. Apai, "Effect of Oxidation States on the Syngas Activity of Transition Metal Oxide Catalysts", American Chemical Society, 191st National Meeting, April 13-18, 1986.